1. Field of the Invention
The present invention relates generally to the control of the printer hammer of an impact printer.
Impact printers generally operate to print characters on a printing substrate by the forceful striking of characters in relief against the substrate via an inked ribbon under the action of a printer hammer. The printer hammer is generally driven by an actuating current whose magnitude determines the impact force. Typically, the magnitude of the hammer actuating current is set to assure a good imprint on a relatively thick substrate, particularly a substrate composed of several sheets of paper which are to be simultaneously printed with the aid of carbon paper.
A forceful striking will not pose a significant threat to the structural integrity of the hammer if the substrate is sufficiently thick to absorb the impact. However, when a relatively thin substrate, e.g. one or two sheets, is being printed on, the substrate may not be sufficiently thick to satisfactorily absorb the impact forces. Then, the hammer may chip or break.
The actuating current for the hammer is normally set at a maximum level in order to adequately drive the keys to print on thick substrates such as multiple carbon copies or the like. In these instances sufficient current must be supplied since the operator is unable to predict beforehand the thickness of the substrate.
It is therefore a primary object of the present invention to protect such printer hammers from excessively forceful driving impact when printing is being effected on a thin substrate.